Dimethyl carbonate, (CH3O)2CO3 finds large industrial applications. It replaces toxic phosgene in the manufacture of polyurethanes and polycarbonates. It finds application as a “green” solvent and as an eco-friendly reagent in methylation, esterification, carbomethoxylation and carbonylation reactions. Dimethyl carbonate is also a potential oxygenate fuel additive replacement to MTBE. Three commercial methods of dimethyl carbonate production are known. Phosgenation of methanol is the route formerly used. Oxidative carbonylation of methanol in liquid or vapor phase using CuCl (U.S. Pat. No. 5,536,864), nitric oxide (UBE) or copper supported on active carbon (U.S. Pat. Nos. 5,183,920 and 5,543,548) catalysts is the second route. The third commercial method (also using CO+O2) developed by UBE industry employs a Pd2+ catalyst and an alkyl nitrite promoter. All these methods produce dimethyl carbonate in high yields but the chemicals used and vent discharges in the processes are toxic or corrosive.
Transesterification of cyclic carbonates with methanol is currently practiced at Asahi Corporation Ltd (U.S. Pat. No. 5,847,189A and 6,479,689B1). Dimethyl carbonate is produced in good yields. This reaction with small exotherm is carried out in the liquid phase without any toxic or corrosive chemicals. But alkelene diol in equimolar amounts is cogenerated. The one-pot reaction of epoxide, carbon dioxide and methanol produces dimethyl carbonate in moderate yields (U.S. Pat. Nos. 7,145,028; 6,607,279B1; 4,434,105 and 6,407,279 B1). Even in this reaction alkelene diol is cogenerated. Reaction of methanol with urea in presence of catalyst leads to dimethyl carbonate. Ammonia is produced as co-product which can be recycled back into urea production (U.S. Pat. Nos. 7,314,947 and 7,074,951). However, the reaction of dimethyl carbonate and ammonia leads to undesired carbamate and isocyanate products. All these processes of dimethyl carbonate synthesis are not atom-efficient. The co-products and their separation affect the economics of the process.
Direct synthesis of dimethyl carbonate by the reaction of methanol with carbon dioxide is the simplest and desirable route. Water is produced as a by-product in this reaction. This reaction is more atom-efficient than the above-said routes. However, yield of dimethyl carbonate is low in this reaction due to thermodynamic limitations. Development of more efficient catalysts that could activate simultaneously carbon dioxide and methanol and overcome the limitations is desirable. Moreover, utilizing carbon dioxide, a greenhouse emission gas, as a raw material in the production of a green chemical would possibly make some positive impact on reducing global warming and carbon dioxide levels in the atmosphere.
U.S. Pat. No. 7,605,285 B2 provides a method and apparatus for simultaneous production of methanol and dimethyl carbonate. Methanol is synthesized by allowing the synthesis gas to react over a catalyst, and dimethyl carbonate is produced by adding carbon dioxide to the methanol, characterized in that carbon dioxide in combustion exhaust gas discharged from a combustion radiator section for heating a reaction tube of the reformer is recovered. Dimethyl carbonate yields are limited by thermodynamic limitations.
US patent 2011/0196167 A1 discloses a method for producing dimethyl carbonate, the method comprises providing effective amounts of methanol and carbon dioxide to a reaction vessel, reacting methanol and carbon dioxide in the presence of a heterogeneous catalyst in the reaction vessel to produce dimethyl carbonate wherein the heterogeneous catalyst provides both acidic sites and basic sites. The catalyst is selected from the group consisting of Rh/Al2O3, Pd/Al2O3, Pt/Al2O3, Ni/Al2O3, Rh/SiO2, Rh/ZSM-5, Rh—K/Al2O3, Ni/SiO2—Al2O3, Mo2C/Al2O3, Pd/V2O5, Pd/TiO2, Pd/V2O5—TiO2, Pd/TiO2—ZrO2, Pt/Al2O3, Re/Al2O3, MoO3/Al2O3, MoO3/ZSM-5, MoO3/SiO2 and combinations thereof. The reaction is performed at ambient temperature and at a temperature from about 80° C. to about 280° C. At 80° C., dimethyl carbonate is the selective product but at higher temperature, formation of significant amount of undesired dimethyl ether was detected.
References may be made to the following literature. Fang and Fujimoto (Appl. Catal. A: Gen. Vol. 142, Year 1996, Page L1) synthesized dimethyl carbonate from methanol and carbon dioxide using methyl iodide and K2CO3 as promoters. Although this reaction was fast, its deactivation was very rapid. Zirconia-based materials with both acidic and basic properties have been used as heterogenous catalysts for this reaction (Tomishige et al., J. Catal. Vol. 192, Year 2000, page 355; Ikeda et al., J. Phys. Chem. B Vol. 105, Year 2001, page 10653; Jiang et al., Appl. Catal. A: Gen., Vol. 256, Year 2003, page 203). The yield of DMC formed was low over these catalysts.
Article titled, “A novel method of direct synthesis of dimethyl carbonate from methanol and carbon dioxide catalyzed by zirconia” by Keiichi Tomishige, Tomohiro Sakaihori, Yoshiki Ikeda, Kaoru Fujimoto in Catalysis Letters, Year 1999, Volume 58, Issue 4, pp 225-229 reports the synthesis of Dimethyl carbonate from methanol and CO2 with high selectivity using ZrO2 catalysts. In this reaction, the amount of dimethyl ether and CO was below the detection limit. Further it reports, the catalytic activity seems to be related to acid-base-pair sites of the ZrO2 surface from the results of temperature-programmed desorption of NH3 and CO2. It also reports the selectivity of DMC formation on Zirconia catalyst as 100% under all the reaction conditions studied.
Article titled, “Promoting effect of phosphoric acid on zirconia catalysts in selective synthesis of dimethyl carbonate from methanol and carbon dioxide” by Yoshiki Ikeda, Tomohiro Sakaihori, Keiichi Tomishige and Kaoru Fujimoto in Catalysis Letters 66 (2000) 59-62 reports the addition of phosphoric acid to zirconia catalysts promoted the activity for dimethyl carbonate synthesis from methanol and carbon dioxide with high selectivity, and the reactions proceeded at much lower temperature on H3PO4/ZrO2 than on zirconia catalysts. It also suggests that the surface acidity enhancement by phosphoric acid contributed to higher activity. The selectivity of DMC formation on H3PO4/ZrO2 is estimated 100%. The catalyst was optimized by employing calcinations temperature of 600° C. and P/Zr ratio of 0.05. However, even with this optimized catalyst the maximum yield of dimethyl carbonate produced at CH3OH:CO2 (milli molar ratio)=192:200, catalyst weight=0.5 g, reaction temperature=170° C. and reaction time=2 h was 0.85 mmol/g catalyst (0.0765 g. DMC/g. catalyst) only which is significantly low for possible commercialization.
In view of the importance of dimethyl carbonate in industrial applications and drawbacks of prior-art processes which include low yield of dimethyl carbonate, formation of undesired dimethyl ether and catalyst deactivation, it is desirable to have a more efficient solid catalyst and a process using the catalyst. The process of the present invention using zirconium phosphonate phosphite or zirconium phosphonate phosphate compound is highly efficient and overcomes the deficiencies of prior-art processes.